The present invention relates to an improvement of wideband antennas with omni-directional radiation, more particularly of antennas of the type described in the patent application WO2005/122332 in the name of the applicant. Antennas of this type are used to receive and/or transmit electromagnetic signals that can be used in the wireless high bit rate communications field, more particularly in the case of wideband pulse regime transmissions of UWB (Ultra Wide Band) type. Such communications are, for example, of types WLAN, WPAN, WBAN (Wireless Local/Personal/Body Area Network).
In pulse regime, the information is sent in a pulse train, for example very short pulses in the order of a nanosecond. This results in a very wide band of frequencies.
Ultra Wideband transmissions, originally reserved for military radar applications, are gradually being introduced into the domain of civil telecommunications. Hence, the frequency band [3.1; 10.6] GHz was recently adopted by the American FCC body to enable the development of UWB communication applications for which the standard is currently being constructed.
Many applications require isotropic antennas, that is with a symmetry of revolution in the radiation pattern. This is particularly the case for applications in which portable products are used, which theoretically have no special fixed position and which must communicate via a UWB wireless link with a point of access. Here, for example products of the type Video Lyra, mobile PCs, etc. are involved. This is also the case for fixed point-to-point applications for which a permanent link is required to be provided in order to obtain a certain quality of (QoS). Indeed, person(s) moving can break the beam between two highly directive antennas and it is preferable to use omni-directional antennas for transmission and/or reception. Here, for example, a video server communicating with a high definition television receiver is involved.
One of the most well known omni-directional antennas is the dipole. As shown on FIG. 1, a dipole comprises two identical arms 101 and 102 of length λ/4 placed opposite each other and differentially supplied by a generator 103. This type of radiating element has been thoroughly studied and used from the beginnings of electromagnetism, mainly for its simplicity of implementation but especially for the simplicity of the mathematic expressions governing its electromagnetic mechanism. Chapter 5 of “Antennas” by J. D. Kraus, Second Edition, Mac Graw Hill, 1988, contains the mathematical expressions explaining the mechanism of this type of radiating element. In particular, the long distance radiated field is maximum in the midperpendicular plane of the dipole (plane xOz in FIG. 1), and its theoretical impedance is around 75Ω. It was originally used in wireline technology for diverse applications such as amateur radio, UHF reception and even more recently in the wireless networks of the WLAN type. Since the advent of printed circuits, its realization has been simplified still further, the antenna now becoming an integral part of the circuit.
The problem related to this type of radiating element is on the one hand its small bandwidth and on the other its supply, which generally disturbs the symmetry of the structure. This leads to a disymmetrization of the near fields and results in a degradation of the far field pattern. Consequently, this is no longer as omni-directional. On the other hand, this type of antenna presents a small bandwidth.
To overcome these disadvantages, the patent application WO 2005/122332 proposes an antenna topology enabling an ultra wide band operation with an omni-directional radiation pattern. This antenna which will be described in more detail hereafter is comprised of two conductive arms placed on a substrate, one of the arms being supplied by a line passing under the other arm and forming a stripline structure.
However, the regulation bodies having imposed extremely low levels for the UWB terminals in the WiFi frequency bands between 4.92 and 5.86 GHz, it is necessary to integrate a filtering structure to this type of antenna. The filtering structures generally proposed are constituted of line-slots realized in the conductor arm(s), as described for example in the patent U.S. Pat. No. 7,061,442. However, the rejection rate as well as the bandwidth are insufficient.